Why can't I delete some system files, even as an administrator?

macOS protects its own files with System Integrity Protection and a sealed, read-only system volume, so even an administrator can't delete or change them. That protects your Mac from malware and accidents.

Those protected files aren't what fills a disk anyway — the space you can reclaim is in your own files, apps and their data, caches, snapshots and backups. Disabling these protections to delete system files is not recommended.